Process for improving grain structure of aluminum silicon alloys



United States Patent 3,466,170 PROCESS FOR IMPROVING GRAIN STRUCTURE OF ALUMINUM SILICON ALLOYS Eckhard Dunkel, Frankfurt am Main, and Wolfgang Thiele, Bad Homburg vor der Hohe, Germany, assignors to Metallgesellschaft Aktiengesellschaft, Frankfurt am Main, Germany No Drawing. Filed Jan. 12, 1967, Ser. No. 608,754 Claims priority, application Germany, Jan. 13, 1966, M 67,990 Int. Cl. C22c 21/04 US. Cl. 75--148 3 Claims ABSTRACT OF THE DISCLOSURE Process for improving modification structure of Al-Si alloys with a content of 5 to 14% of Si and optionally Mg and/or Zn or Cu by introduction of Sr and/or Ba in a quantity of 0.001 to 2.0% into the molten alloy prior to casting whereby a long lasting modification effect is obtained. The Sr and/ or Ba can be added in metallic form as such or in the form of alloys or in the form of their compounds. Optionally Be may also be added to the molten Sr and/or Ba containing alloy in a quantity of 0.001 to 2.0%.

FIELD OF INVENTION The invention relates to a process for improving the grain structure of aluminum silicon alloys by treatment of molten baths of such alloys by addition thereto of material which modifies the melt whereby a fine structure of the solidified alloy produced from such molten baths is obtained. This modification effect is of long duration.

It is already known that a fine eutectic grain structure can be achieved in Al-Si alloys with 5 to 13% of Si by addition of suitable substances or materials to such alloys when in the molten state, so as to improve the mechanical properties of such alloys. According to the proposals of the prior art, this is achieved by treatment of the Al-Si alloys in molten state with a number of alkali metals and alkaline earth metals or mixtures of such metals or their compounds. Beryllium has also been suggested for this purpose. It also is known that a modification effect can be achieved in Al-Si alloys containing 5 to 13% of Si by the addition of calcium which has a longer effect on the molten alloy than sodium. Furthermore, Al-Si alloys which have a small strontium content and good technological properties have become known as materials for the construction of highly stressed pistons and cylinder heads. The major portion of the known processes for the improvement of Al-Si alloys, especially, the proposals, which have been known for over forty years, which concern the use of alkaline earth metals could not find acceptance in the practice and the art did not accord any practical significance to such proposals as they were considered paper state of the art.

Only modification with sodium or sodium yielding compounds could find acceptance in casting practice. However, it previously was not possible to maintain the modification effect for a sufficient period of time by a single addition of sodium. For this reason, for example, in chill mold casting processes, Al-Si alloy melts periodically have to be given repeated modification treatments. This disturbs the continuity of the casting operation.

The regulation of the correct degree of modification in Al-Si alloys according to experience is not always sufficiently certain and reproducible, as errors in dosing and procedure during the introduction of the metallic sodium or sodium yielding substances into the melts cannot be avoided and lead to serious deficiencies in or the discord of castings produced from the melts. In addition,

very narrow limits are set for the use of degassing or other melt purifying agents in sodium refined Al-Si alloy melts in view of the volatility and high activity of the sodium. Al-Si alloy melts which have been modified with calcium do exhibit a modification effect, which is less than that attained with sodium but of longer duration. Al-Si alloys which have been modified with calcium, however, have very serious drawbacks. For instance, the calcium addition promotes the tendency of the eutectic Al-Si alloys to oxidize to a great degree and the alloys which have been treated with calcium have an unattractive appearance because of their dirty gray surface color (normally treated Al-Si alloys exhibit a metallic lustre) and in addition exhibit a most undesirable oxide skin formation near the gates of castings which in foundry practice is called elephant skin. The decisive disadvantage of calcium modified Al-Si alloys, however, is to be seen in that only very rapidly solidified cast parts, for instance, in chill molded castings with a wall thickness of 3 mm. exhibit a pseudo modified structure whereas a lamellar structure occurs at locations of local overheating. The slower the solidification takes place the greater the occurrence of zones of lamellar structure which results in the increased occurrence of shrinkage faults which are typical for lamellar structures. These shrinkage faults are extremely disadvantageous and cannot be avoided by the usual supply measures.

Al-Si alloy melts which have been modified with calciurn, the modification effects are retained with one remelting procedure, but here again the oxidic influences are increased.

Therefore, when seen as a whole, the modification of Al-Si alloys with calcium result in more disadvantageous effects so that the advantageous effect of modification action of long duration steps into the background. As a consequence this process also, despite its seeming advantages could not be introduced in practice. To the contrary care has been taken in aluminum metallurgy to remove calcium from Al-Si alloys.

SUMMARY OF THE INVENTION It is an object of the invention to provide a process for modification Al-Si alloys which possesses the advantages of the prior art processes but at the same time avoids their disadvantages and furthermore provides a material suitable for the construction of highly stressed structural parts.

The invention therefore relates to a process achieving a long lasting modification effect in Al-Si alloys by the introduction of an alkaline earth metal in a small quantity into Al-Si alloy melts. According to the invention this is effected by adding strontium and barium individually or in combination in a quantity of 0.001 to 2.0 wt. percent, preferably, 0.05 to 0.2 wt. percent, to Al-Si alloy melts with 5 to 14 wt. percent of Si and if desired contents of Mg and/or Zn/or Cu and the remainder Al and in addition may contain the usual impurities.

The process according to the invention is not only suited for binary Al-Si alloys but also Al-Si alloys with other alloy components. The Al-Si alloys which, for example, are suited for the practice of the invention are heat hardenable magnesium containing Al-Si alloys of the type of Al-Si-10% Mg, Al-Si-7% Mg and Al-Si-S% Mg and cold hardening Zn and Mg containing alloys of the type Al-Si-8% Zn-10% Mg. The usual impurities which may be contained in the Al-Si-alloys, for instance, are Cu, Mn, Fe, Ni, Ti, Pb or Sn which do not have any disadvantageous action in a total quantity of 8%. Alloys also suited are heat or cold hardenable copper containing Al-Si alloys of the type Al-10%Si-4% Cu. The usual impurities which may be contained in these alloys are Mg,

Zn, Mn, Fe, Ni, Ti, Pb or Sn having no disadvantageous action in a total quantity of 8%.

The additions of strontium and/or barium can be effected in the form of pure metal or alloys of such metals which in some instances may be of advantage. Suitable alloys, for instance, are Al-Sr alloys with 7 wt. percent Sr or Al-Ba alloys with wt. percent Ba. The introduction is effected in a manner known per se, for instance, in foundry operation the Al-Si alloy, after normal smeting and melt purification, is treated with an Al-Sr or Al-Ba alloy by immersion of the latter therein, pouring over the latter or stirring the latter therein to effect alloying and then casting the treated melt to form castings in the usual manner.

The process according to the invention, however, also may be advantageously carried out by adding the Sr and/or Ba to the Al-Si alloy melt in the form of Sr and/or Ba compounds. Expediently their oxides, sulfates or carbonates are employed for this purpose. The melting points of such compounds and therefore also their activity can be improved (that is, the Sr and/ or Ba are more easily taken up by the Al-Si alloy melt) by the addition of halogen salts of the alkali metals or other alkaline earth metals. The salt mixtures may also be used in pressed form and can also contain finely divided reducing metals such as Al or Mg whereby the liberation of the strontium and/or barium and their take up in the Al-Si alloy melt is facilitated.

It was furthermore found that the effect of the Sr and/or Ba addition according to the invention can be assisted by the addition of Be in a quantity of 0.001 to 2.0 wt. percent, preferably, 0.05 to 0.2 wt. percent. The beryllium addition can be simultaneous with or before or after the addition of the Sr and/ or Ba. Whereas beryllium by itself has no modification effect in Al-Si aloys nor has a noticeable or noteworthy effect on the oxidation of Sr and Ba free Al-Si alloys, the simultaneous presence of Be and Sr and/or Ba provides a synergistic effect. The oxidation of Sr is hindered by Be so that no Sr is lost in the melt and the modification effect is maintained for a still longer period of time.

This action of the Be addition is especially noticeable in Al-Si alloys modified with Ca. Al-Si a loy melts with 5l4% Si and other alloying components such as Mg, Zn, Cu, Ni, Mn or Fe in addition to usual impurities and the remainder Al which contain 0.001 to 2 wt. percent of Ca and 0.001 to 2 wt. percent of Be have a reduced tendency to oxidize and can be cast free of shrinkage faults and oxidic inclusions as well as of the so-called formation of elephant skin. As a result thereof, they also have noticeably better mechanical properties than Al-Si alloys treated with Ca but free to Be, so that this process first becomes capable of use in foundry practice by the use of beryllium.

The process according to the invention has a number of advantages. The modification elfect achieved with Sr and/or Ba, if desired, in conjunction with Be, is retained over a long period of time. The effect for instance is retained for at least 2 hours and up to 10 hours and in some instances can be retained beyond this range depending upon the quantity of the addition and the temperature at which the molten Al-Si alloys are maintained. It is possible thereby to process large charges of Al-Si alloys either continuously or discontinuously without the necessity of interrupting the processing of the charge to permit further additions of modifying agents. In addition, the modification effect is not lost upon solidification and remelting of the charge but rather the remelting causes an improvement in the mechanical properties of the Al-Si alloys which have been treated according to the invention as the silicon rich phase of the eutectic separates out in a finer condition and more strongly rounded off than in the castings of the alloy after the first smelting. The Al-Si alloys which have been modified with Sr or Ba in addition have excellent flow characteristics which are better than those obtained in Al-Si alloys modified with Na and about correspond to those of the untreated Al-Si alloys.

The Al-Si alloys which have been treated according to the invention can, with especial advantage, also be subjected to degasifying agents or other smelt purifying agents for the production of pore free castings, as such Al-Si alloy melts can be treated with inert or reactive gases for the purpose of eliminating a harmful gas content without the fine grained nature of the alloy being lost on casting.

The Al-Si alloys which have been treated according to the invention are especially well suited for use as construction materials for the production of highly stressed parts which require a high elongation, for example, parts which have an elongation over 10%, a tensile strength of over 19 kg./mm. and a yield point of over 9 kg./mm. In addition, Al-Si alloys treated according to the invention can also be used in the construction of parts requiring a high tensile strength and high yield point and must have a tensile strength of over 36 kg./mm. a yield point of 32 kg./mrn. with an elongation of 3%. For instance, Al-Si alloys, such as, for example, one of 7 wt. percent Si, 0.65 wt. percent Mg, 0.1 wt. percent Sr and the balance Al, can be used with special advantage for the production of highly stressed machine and construction parts of complicated shapes, such as, for example, crankcases of motors, undercarriages and stressing gears of automobiles and aircraft, and warp beams of textile machines.

In addition, the Al-Si alloys treated according to the invention, especially when they have been subjected to age hardening, have good further processing properties. They therefore are suited for parts which must be subjected to a fine finishing operation involving a shaving removal action, such as, for example, valve housings and armature parts, the valve seats of which must exhibit a smooth surface.

THE PREFERRED EMBODIMENTS The process according to the invention is illustrated by the following examples of preferred embodiments in which the proportions are given by weight unless specified otherwise.

Example 1 An alloy of the composition 7% Si, 0.65% Mg and the remainder Al was alloyed with 0.1% of Sr at 740 C. using a prealloy of Al containing 5% Sr. The resulting alloy was held for 2 hours in the molten condition and then cast into sand mold and chill mold rods. The remainder of the alloy was cast into pigs. The rods were annealed for 8 hours at 535 C., quenched in water, stored for 4 hours at room temperature and then age hardened for 12 hours at C. The rods thus obtained had the following mechanical properties.

Tensile Yield Brinell strength, point, Elongation, hardness kg/mm. kg./m1n. percent kg./nun.

Sand m0ld 27. 8 24. 3 1. 3 96. 3 Chill mold 31. 5 24. 3 6. 5 97. 2

The alloy which had been cast into pigs was remelted and cast into rods with sand molds and chill molds and the rods heat treated as above. The rods thus obtained had the following mechanical properties.

Tablets consisting of a mixture of 67% SrCO and 33% Mg powder were introduced into 8.5 kg. of a melt 5 of an Al-Si alloy containing 12% Si at 700 C. The strontium content of the mixture introduced was 0.3%. Samples cast at 720 C. after the treated melt had been held for 4 hours at 720 C. exhibited a very good modified structure.

Example 3 In order to ascertain the flow qualities and Al-Si alloy with 9.7% Si and the usual impurities was prepared. Three samples of the same quantity were cast into a special mold according to Ph. Schneider at 720 C. and the length of the spiral of the alloy cast measured.

The first sample, which was given no modification treatment, resulted in a spiral casting 103 cm. long. The second sample which was treated with an A1 prealloy containing 5% of strontium to provide a 0.1% content in the melt resulted in a spiral casting 104 cm. long. The third sample which was treated with a usual commercial Na modification salt resulted in a spiral casting 81 cm. long.

Example 4 Tensile Yield Brinell strength, point, Elongation, hardness, kg/mm. lrgJmm. percent kgJmmfl Sand mold 25. 8 24. 0.5 106 Chill mold 30.6 25.4 3.4 107 Example 5 0.10% metallic strontium were alloyed with an alloy of the composition Percent Zn 9.7 Mg 0.36 Al Remainder at 760 C. The molten alloy was held at this temperature for 4 hours and was then cast in chill molds. After 12 days cold hardening the rods thus obtained had the following mechanical properties Tensile strength "kg/mm? 30.6

Yield point "kg/mm? 21.0

Elongation Percent 2.8

Brinel hardness kg./mm. 112

Example 6 0.10% metallic strontium were alloyed with an alloy of the composition Percent Si 8.6

A1, remainder at 760 C. The molten alloy was held at this temperature for 4 hours and was then cast in chill molds. The rods thus obtained had the following mechanical properties:

Tensile strength kg./m.m. 32.4 Yield point kg./mm. 15.8 Elongation Percent 4.2 Brinell hardness "kg/mm? 112 Example 7 An alloy of the composition Percent Si 10.2

Al, remainder was alloyed with 0.12% calcium and 0.08% beryllium at 740 C. using aluminum prealloys containing 10% Ca and aluminum prealloys containing 3.8%. Be. The resulting alloy was held for min. at 720 C. and then cast in chill molds. The rods were annealed for 12 hours at 525 C. quenched in water and then age hardened for 12 hours at C. The rods thus obtained had the following mechanical properties Tensile strength kg./mm. 35.2 Yield point kg./mm. 29.8 Elongation Percent 6.9 Brinell hardness kg./mm. 115

The rods were free of oxide skins.

We claim:

1. A process for attaining a long lasting modification effect in aluminum silicon alloy-s comprising forming a melt containing aluminum and from 5 to 14 wt. percent silicon, and adding to said melt strontium or barium salts mixed with a finely divided reducing metal selected from the group consisting of aluminum and magnesium.

2. A process as in claim 1, said strontium or barium comprising from 0.001 to 2.0 wt. percent.

3. A process as in claim 2, in which said aluminum silicon alloy contains magnesium or zinc plus customary impurities.

References Cited UNITED STATES PATENTS 1,657,389 1/1928 Gwyer et al. 75-148 2,013,926 9/1935 Pacz 75-68 3,128,176 4/1964 Martin 75-148 3,199,979 8/1965 Martin 75-148 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

